Microwave ablation generator control system

ABSTRACT

A microwave energy delivery and measurement system, including a microwave energy source configured to delivery microwave energy to a microwave energy delivery device, a measurement system configured to measure at least one parameter of the microwave energy delivery device and a switching network configured to electrically isolate the microwave energy source and the measurement system. The measurement system is configured to actively measure in real time at least one parameter related to the microwave energy delivery device.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a Continuation Application claiming thebenefit of and priority to U.S. application Ser. No. 12/242,102, filedon Sep. 30, 2008, by Joseph D. Brannan et al., entitled “MICROWAVEABLATION GENERATOR CONTROL SYSTEM”, the entire contents of which beingincorporated by reference herein.

BACKGROUND

1. Technical Field

The present invention relates to systems and methods for performing amedical procedure, wherein the medical procedure includes the generationand transfer of energy from an energy source to a dynamically changingdevice and, more particularly, efficient transfer of energy through amicrowave energy delivery, measurement and control system.

2. Description of Related Art

During microwave ablation procedures, the electrical performance of amicrowave antenna probe changes throughout the course of an ablationtreatment. The change in performance may be due to the device or due tochanges in tissue properties. The ability to observe parametersindicative of changes in antenna property, antenna performance or tissueproperties changes during ablation greatly aids in the understanding ofmicrowave ablation.

For example, measuring antenna impedance is a common method fordetermining antenna performance and/or a change in an antenna property.Microwave systems are typically designed to a characteristic impedance,such as, for example, 50 Ohms, wherein the impedance of the generator,the delivery system, the ablation device and tissue are about equal tothe characteristic impedance. Efficiency of energy delivery decreaseswhen the impedance of any portion of the system changes.

With low frequency RF systems impedance can easily be determined bymeasuring the delivered current at a known voltage and calculatingtissue impedance using well known algorithms. Obtaining accuratemeasurements of tissue impedance at microwave frequencies is moredifficult because circuits behave differently at microwave frequency.For example, unlike an electrode in an RF system, an antenna in amicrowave system does not conduct current to tissue. In addition, othercomponents in a microwave system may transmit or radiate energy, like anantenna, or components may reflect energy back into the generator. Assuch, it is difficult to determine what percentage of the energygenerated by the microwave generator is actually delivered to tissue,and conventional algorithms for tissue impedance are inaccurate.

Therefore, other methods of measuring impedance are typically used in amicrowave system. One well known method is an indirect method usingmeasurements of forward and reflected power. While this is a generallyaccepted method, this method can also prove to be inaccurate because themethod fails to account for component losses and depends on indirectmeasurements, such as, for example forward and reflected powermeasurements from directional couplers, to calculate impedance. Inaddition, this method does not provide information related to phase, acomponent vital to determining antenna impedance.

One alternative method of measuring impedance in a microwave energydelivery system is by determining broadband scattering parameters.Capturing antenna broadband scattering parameters periodicallythroughout a high power ablation cycle necessitates the use of equipmentthat requires precise calibration. Unfortunately, this equipment isprone to damage by high power signals and the microwave energy deliverysystem typically needs to be reconfigured to accommodate and protectsuch equipment.

The present disclosure describes a Microwave Research Tool (MRT) thatincludes a system to measure impedance in a microwave energy deliverysystem by direct and indirect methods including a system to measurebroadband scattering parameters.

SUMMARY

The present disclosure relates to a microwave energy delivery andmeasurement system for use in testing microwave energy systems anddevices and for use in performing medical procedures. In one embodiment,the microwave energy delivery and measurement system includes amicrowave energy source configured to delivery microwave energy to amicrowave energy delivery device, a measurement system configured tomeasure at least one parameter of the microwave energy delivery deviceand a switching network configured to electrically isolate the microwaveenergy source and the measurement system. The measurement system isconfigured to actively measure a parameter related to the microwaveenergy delivery device, such as, for example, voltage, current and/orimpedance.

In another embodiment the active measurement system of the microwaveenergy delivery and measurement system further includes a processorconfigured to control the active measurement system and a frequencygenerator configured to provide a variable frequency signal to themicrowave energy delivery device. The active measurement system may beconfigured to measure at least one parameter related to the variablefrequency signal delivered to the microwave energy delivery device. Theprocessor may be configured to determine at least one parameter relatedto the microwave energy delivery device.

In yet another embodiment, the measurement system may include a passivemeasurement system. The passive measurement system may include a dualdirectional coupler configured to provide a signal related forward powerand/or reflected power.

In yet another embodiment, the switching network of the microwave energydelivery and measurement system is configured to connect the microwaveenergy delivery device to the microwave generator in a first conditionand connect the microwave energy delivery device to the measurementsystem in a second condition. The switching network may dynamicallyswitch between the first and second conditions.

The switching network includes a first switch and a second switch. Thefirst switch is configured to switch energy from the microwave generatorbetween a first resistive load and a circulator. The second switch isconfigured to connect the microwave energy delivery device between thecirculator and the measurement system. The circulator passes a signalfrom the first switch to the second switch and passes a signal from thesecond switch to a ground potential through a second resistive load.

The first condition includes a first electrical connection between themicrowave generator and the circulator through the first switch and asecond electrical connection between the microwave energy deliverydevice and the circulator through the second switch. The microwavesignal is passed from the microwave generator, through the firstelectrical connection to the circulator, from the circulator through thesecond electrical connection and to the microwave energy deliverydevice.

The second condition includes a third electrical connection between themicrowave generator and the first resistive load through the firstswitch; and a fourth electrical connection between the microwave energydelivery device and the active measurement system through the secondswitch. The active measurement system is configured to measure aparameter related to the performance of the microwave energy deliverydevice.

In a further embodiment the first switch is a variable attenuatorconfigured to proportionate energy from the signal generator between aterminator resistor and an amplifier.

In still yet another embodiment, the measurement system includes atleast one input configured to receive a first signal related to theenergy delivered to the microwave energy delivery device from themicrowave energy source and an output configured to provide ameasurement signal to the microwave energy delivery device. The firstsignal may be related to forward power and/or reflected power and themeasurement signal may be related to a parameter of the microwave energydelivery device, such as, for example, voltage, current, and/orimpedance.

In a further embodiment the measurement system may include a processorconfigured to control the measurement signal and to process the signalreceived by the at least one input. The processor may vary the frequencyof the measurement signal and determine a parameter related to themicrowave energy delivery device at one or more frequencies.

In yet another embodiment, the microwave energy delivery and measurementsystem may include a first switch, a first resistive load connectedbetween a ground potential and the first switch, a second switch, asecond resistive load connected between a ground potential and thesecond switch, a circulator connected between the first and secondswitches, and a third resistive load connected between a groundpotential and the circulator. The first switch directs the microwaveenergy between the first resistive load and the circulator, thecirculator directs microwave energy from the first switch to the secondswitch and directs energy from the second switch to the third resistiveload, and the second switch connects a microwave energy delivery deviceto one of the circulator and the measurement system.

In a first condition the microwave energy from the microwave energysource is supplied to the microwave energy delivery device through thefirst switch, the circulator and the second switch and the measurementsystem is isolated by the second switch. In a second condition themeasurement system connects to the microwave energy delivery devicethrough the second switch and the first switch and the second switchesisolate the microwave energy source from the measurement system.

The present disclosure relates to an intermittent microwave energydelivery system for use in testing microwave energy systems and devicesand for use in performing medical procedures. In one embodiment, theintermittent microwave energy delivery system includes a microwaveenergy source configured to provide a continuous microwave energysignal, an energy delivery network configured to intermittently transmita portion of the continuous microwave energy signal, a resistive loadconfigured to dissipate the microwave energy signal; and a switchingnetwork configured to switch the continuous microwave energy signalbetween the microwave energy network and the resistive load. Thecontinuous microwave energy signal is time proportioned between theenergy delivery network and the resistive load.

The switching network may include a high speed switch to switch themicrowave energy signal between the energy delivery network and theresistive load. The high speed switch may transition from deliveringenergy to the energy delivery network to the resistive load in about 360ns and may transition from delivering energy to the resistive load tothe energy delivery network in about 360 ns.

In another embodiment the switching network is configured to vary theduty cycle of the signal delivered to the energy delivery networkbetween about 10% on-time to about 90% on-time. The system may furtherinclude a processor configured to vary the duty cycle of the switchingnetwork. The duty cycle of the switching network may be determined by aparameter such as, for example, a forward power measurement, areflective power measurement and/or a temperature measurement.

In a further another embodiment the switching network includes avariable attenuator configured to receive the continuous microwavesignal from the microwave energy source, a resistive load connectedbetween the variable attenuator and a ground potential and an amplifier.The variable attenuator is configured to proportionate the continuousmicrowave signal from the microwave energy source between the resistiveload and the amplifier and the amplifier amplifies the microwave signalfrom the variable attenuator and supplies the amplified signal to theenergy delivery network.

The present disclosure relates to a system, apparatus and method fordissipating standing waves in a microwave energy delivery system. In oneembodiment, a system for dissipating a standing wave includes amicrowave energy source configured to intermittently delivery microwaveenergy as a periodic microwave signal an energy delivery networkconfigured to transmit the periodic microwave signal and a circuitconnected between the microwave energy source and the energy deliverynetwork. The circuit is configured to pass the periodic microwave signalfrom the microwave energy source to the energy delivery network when theperiodic microwave signal is present and to dissipate standing waveswhen the periodic microwave signal is absent.

In a further embodiment the circuit includes a first resistive load anda circulator configured to direct the periodic microwave signal from themicrowave energy source to the energy delivery network. The circulatoris also configured to direct energy from the energy delivery network tothe first resistive load, the first resistive load connected between thecirculator and a ground potential. The first resistive load dissipateenergy reflective from the energy delivery network when the periodicmicrowave signal is in a high energy condition and dissipates residualenergy when the periodic microwave signal is in a low energy condition.

In yet another embodiment, the system for dissipating a standing wavealso includes a microwave energy delivery device, a network analyzer, asecond resistive load, connected between the transfer switch and aground potential, and a transfer switch connected between thecirculator, the microwave energy delivery device, the second resistiveload and the network analyzer. The transfer switch, in a firstcondition, connects the network analyzer to the microwave energydelivery device and the circulator to the second resistive load. Thetransfer switch, in a second condition, connects the circulator to themicrowave energy delivery device and the network analyzer to the secondresistive load. The transfer switch electrically isolates the networkanalyzer from the microwave energy source. In a further embodiment themicrowave energy delivery device is a medical device.

The first transfer switch, in a first condition, passes a testing signalfrom the network analyzer to the microwave energy delivery device. In asecond condition the first transfer switch passes a microwave energysignal from the microwave energy source to the microwave energy deliverydevice.

In yet another embodiment of the present disclosure an apparatus fordissipating standing waves in a microwave energy delivery systemincludes a circulator configured to direct a periodic microwave signalfrom a microwave energy source to the an energy delivery network andconfigured to direct energy from the energy delivery network to a firstresistive load wherein the first resistive load is connected between thecirculator and a ground source, the first resistive load furtherconfigured to dissipate or shunt residual energy through the firstresistive load. The first resistive load dissipates energy reflectivefrom the energy delivery network when the periodic microwave signal ispresent and dissipates residual energy in the system when the periodicmicrowave signal is absent.

A method of dissipating standing waves in a microwave energy deliverysystem is also disclosed and includes the steps of: providing amicrowave energy source configured to generate a continuous microwaveenergy signal; time-proportioning the continuous microwave energy signalbetween an energy delivery network and a load resistor connected to aground potential, the energy delivery network configured tointermittently transmit a portion of the continuous microwave energysignal; dissipating reflective energy when the energy delivery networkis receiving the microwave energy signal; and dissipating standing waveswhen the energy delivery network is not receiving the microwave energysignal.

The present disclosure relates to a microwave system calibrationapparatus including an antenna portion configured to deliver microwaveenergy to tissue, a transmission line portion configured to receive amicrowave energy signal from a microwave source and to selectivelydeliver the microwave energy signal to the antenna portion and aswitching mechanism connected between the antenna portion and thetransmission line portion. The transmission line includes an innerconductor having a length, an outer conductor concentrically surroundingthe inner conductor along the length and a dielectric materialseparating the inner and outer conductors. The inner conductor or theouter conductor of the transmission line portion is electricallyconnected to the antenna. The switching mechanism is configured toelectrically disconnect the transmission line portion from the antennaportion in a first condition and further configured to connect the innerconductor to the outer conductor in a second condition.

The switching mechanism may further include an internal antenna circuitwith predetermined circuit parameters. In the second condition the innerconductor connects to the outer conductor through the internal antennacircuit. The impedance of the internal antenna circuit may be about 50ohms The microwave energy source controls the operation of the switchingmechanism.

In another embodiment a calibrating microwave energy delivery systemincludes a microwave energy source configured to supply a microwaveenergy signal and a microwave system calibration apparatus. Themicrowave system calibration apparatus includes an antenna portion, atransmission line portion and a switching mechanism connected betweenthe antenna portion and the transmission line portion. The antennaportion is configured to deliver microwave energy to tissue. Thetransmission line portion receives a microwave energy signal from amicrowave energy source and selectively deliver the microwave energysignal to the antenna portion. The transmission line portion includes aninner conductor having a length, an outer conductor surrounding theinner conductor at lease partially along the length and a dielectricmaterial separating the inner and outer conductors. The inner conductoror the outer conductor of the transmission line portion electricallyconnects to the antenna. The switching mechanism includes a first switchconfigured to electrically disconnect the transmission line portion fromthe antenna portion and a second switch configured to connect the innerconductor to the outer conductor through an internal antenna circuit.The switching mechanism connects the transmission portion to the antennaportion, the internal antenna circuit or an open circuit.

In a further embodiment the microwave energy source connects to, andcontrols the operation of the switching mechanism.

A method of calibrating a microwave system is also disclosed andincludes the steps of: providing a calibrating microwave deliverydevice; connecting the calibrating microwave energy delivery device to amicrowave source; performing an open circuit test: measuring at leastone parameter related to the open circuit test; performing a closedcircuit test; measuring at least one parameter related to the closedcircuit test; and determining at least one calibration parameter relatedto the antenna portion of the calibrating microwave energy deliverydevice. The open circuit test is performed by activating a first switchin the switching mechanism of the calibrating microwave energy deliverydevice, wherein the first switch produces a signal open proximal theantenna portion. The closed circuit test is performed by activating asecond switch in the switching mechanism of the calibrating microwavedelivery device, wherein the second switch connects the inner conductorto the outer conductor through an internal antenna circuit.

The present disclosure relates to a microwave energy delivery andmeasurement system including a microwave energy source configured todelivery microwave energy, a measurement system, a switching networkconfigured to connect the microwave energy delivery device between themicrowave energy source and the measurement system, a tuner connectedbetween the switching network and the microwave energy delivery deviceand a control system. The tuner adjusts the circuit impendence of themicrowave energy delivery device based on a tuner control signal. Thecontrol system is configured to receive data from the measurementsystem, determine an impedance mismatch between the microwave energydelivery device and the microwave energy source and provide the controlsignal to the tuner. The measurement system includes an analog inputconfigured to receive a first signal related to the energy delivered bythe microwave energy source and an analog output configured to produce asecond signal configured to drive the microwave energy delivery device.A parameter of the second signal is related to a property of themicrowave energy delivery device.

In one embodiment the first signal received by the analog input isforward power, reflected power or temperature. The second signalproduced by the analog output is an RF signal or a microwave signal.

The switching network electrically isolates the microwave energy sourceand the measurement system. The microwave energy source may include amicrowave generator configured to generate a microwave signal and afirst switch configured to receive the microwave signal from themicrowave generator. The first switch directs the microwave signal to aload resistor connected to a ground potential or the switching network.

In a further embodiment the switching network further includes a secondswitch configured to connect the microwave energy delivery device to themeasurement system and the microwave energy system. The second switchprovides electrical isolation between the microwave energy deliverysystem and the microwave generator.

In yet another embodiment the control system connects to, and controlsthe operation of the tuner. The control system may dynamically adjustthe tuner during energy delivery.

In yet a further embodiment the data received from the control system isforward power, reflected power or tissue impedance. The data receivedmay also include current, voltage, frequency or impedance. The controlsystem may perform at least one adjustment of the tuner based on theimpedance mismatch between the microwave energy delivery device and themicrowave energy source.

The present disclosure relates to an apparatus for calibrating amicrowave energy delivery device including a body defining a chamberportion therein, the chamber portion configured to receive a portion ofa microwave energy delivery device and the body is configured to absorbenergy transmitted by the microwave energy delivery device at apredetermined absorption rate.

The chamber partially surrounds the microwave antenna of the microwaveenergy delivery device. The chamber is formed by the body is an elongatecylindrical chamber, the elongate cylindrical chamber adapted to receivethe microwave antenna of the microwave energy delivery device.

In another embodiment the chamber is configured to engage the microwaveantenna of the microwave energy delivery device within the chamber.

The body further includes a first body portion configured to receive andposition the microwave energy delivery device and a second body portionconfigured to engage the first body portion and form the chambertherebetween. The first body portion and the second body portion may behingedly engaged. The first and second body portions may include alocking mechanism that locks the calibration device to the microwaveenergy delivery device. The locking mechanism may be a clip, a latch, apin, a locking hinge, a self closing hinge, a magnetic lock or anelectronic closure mechanism.

In yet another embodiment the body may includes a positioner to positionthe microwave energy delivery device in a fixed position relative to thechamber. The positioner on the body may correspond to a substantiallysimilar interface on the microwave energy device. The positioner and theinterface may mate with each other to position the microwave energydelivery device in a fixed position relative to the chamber. Thepositioner may be recessed portion of the body and the interface may bea raised portion of the microwave energy delivery device. The recessedportion and the raised portion mate together and position the microwaveenergy delivery device.

The first body portion and the second body portion surround a portion ofthe microwave energy delivery device in a first condition and are spacedrelative to a portion of the microwave energy delivery device in asecond condition.

A system for calibrating a microwave energy delivery device is alsodisclosed and includes a microwave generator configured to deliver amicrowave energy signal to a microwave energy delivery device and amicrowave system calibration apparatus. The microwave system calibrationapparatus includes a body defining a chamber portion therein, thechamber portion configured to receive a portion of the microwave energydelivery device. The body configured to absorb microwave energytransmitted by the microwave energy delivery device at a predeterminedabsorption rate. The microwave generator measures a measured parameterrelated to the microwave energy signal delivered to the microwave energydelivery device and determines at least one calibration parameterrelated to the calibration of the microwave energy delivery device.

The chamber in the system may partially surround the microwave antennaof the microwave energy delivery device. The chamber, formed by thebody, may be an elongate cylindrical chamber adapted to receive themicrowave antenna of the microwave energy delivery device. The chambermay engage the microwave antenna of the microwave energy delivery devicewithin the chamber.

The measured parameter may be forward power, reflected power ortemperature and the calibration parameter may be phase, frequency orimpedance.

In another embodiment of the system the microwave generator maydetermine engagement of the microwave energy delivery device with themicrowave system calibration apparatus.

A method of calibrating a microwave energy delivery system is alsodisclosed and includes the steps of: connecting a microwave energydelivery device to a microwave generator; positioning the microwaveenergy delivery device in a chamber defined in a microwave energycalibration apparatus; delivering microwave energy to the microwaveenergy delivery device; measuring at least one measured parameterrelated to the energy delivery; determining at least one calibrationparameter related to the calibration of the microwave energy device andutilizing the calibration parameter in a subsequent energy delivery.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of a microwave energy delivery,measurement and control system in an energy delivery mode according toan embodiment of the present disclosure;

FIG. 2 is a state machine functional block diagram of the microwaveenergy delivery, measurement and control system of FIG. 1;

FIG. 3 is a switch control state machine for the microwave energydelivery, measurement and control system including a precision networkanalyzer;

FIG. 4. is a functional block diagram of a precision network analyzerincluding passive and active measurements;

FIG. 5 is a functional block diagram of a microwave energy delivery,measurement and control system including an impedance tuner;

FIG. 6 is a switch control state machine for the microwave energydelivery, measurement and control system including a precision networkanalyzer, CPU and a tuner;

FIG. 7 is a functional block diagram of a microwave energy delivery,measurement and control system according to another embodiment of thepresent disclosure;

FIG. 8A is a schematic representation of an ablation device for use incalibrating the microwave energy delivery, measurement and controlsystem of the present disclosure;

FIG. 8B is a cross-sectional schematic representation of the ablationdevice and switching mechanism for calibrating the microwave energydelivery device;

FIG. 8C is an electrical schematic of the switching mechanism of FIG.8B;

FIG. 9A is a schematic representation of a stand-alone calibrationdevice for use in calibrating the microwave energy delivery, measurementand control system of the present disclosure; and

FIG. 9B is a schematic representation of a interfacing calibrationdevice for use in calibrating the microwave energy delivery, managementand control system of the present disclosure.

DETAILED DESCRIPTION

Detailed embodiments of the present disclosure are described herein;however, it is to be understood that the disclosed embodiments aremerely exemplary and may be embodied in various forms. Therefore,specific structural and functional details disclosed herein are not tobe interpreted as limiting, but merely as a basis for the claims and asa representative basis for teaching one skilled in the art to variouslyemploy the present disclosure in virtually any appropriately detailedstructure.

Referring to FIG. 1, a Microwave Research Tool (MRT) including ameasurement and control system for use in performing a medical procedureor medical procedure testing, employing embodiments of the presentdisclosure is generally designated 100. MRT 100 may provide all thefunctionality of a microwave generator typically used to delivermicrowave energy in a medical procedure but with improved functionalityas described herewithin. MRT 100 includes individual components, asillustrated in FIG. 1, or the functionality of individual components maybe combined or included in one or more components. Components areinterconnected with suitable cables and/or connectors.

MRT 100 includes a microwave energy delivery system, a measurementsystem and a supervisory control system. Each system is describedindividually although each system may share common components asdiscussed hereinbelow.

The microwave energy delivery system includes a signal generator 105capable of generating and supplying a high frequency microwave signal toan amplifier 110. Signal generator 105 may be a single frequencygenerator or may include variable frequency capability. Signal generator105 may also be capable of providing a signal including two or morefrequencies wherein the device under test 115 (DUT) resonates at two ormore frequencies. Supervisory control system may control various aspectsof the signal generator 105 such as, for example, the signal deliverytiming, the frequency (or frequencies) of the output and the phase ofthe signal.

Amplifier 110 receives and amplifies the signal from the signalgenerator 105 to a desirable energy level. Amplifier 110 may be a singleor multi-stage amplifier 110 and may include one or more signalconditioning circuits or filters (not shown) such as, for example, alow, high or bandpass circuits. Amplifier 110 gain may be fixed orcontrolled by a suitable controller, such as, for example, a controlalgorithm in the supervisory control system, a central processing unit120 (CPU) or by manual adjustment (not shown).

Amplifier 110 supplies a continuous, amplified microwave signal to a hotswitch relay 125. Hot switch relay 125 is controlled by the supervisorycontrol system or CPU 120 and switches the amplified microwave signal toone of an amplifier burn-off load resistor 130 and a circulator 135. Thehot switch relay 125 in Position A delivers energy to the DUT 115through the circulator 135. The hot switch relay 125 in Position Bdelivers energy away from the DUT 115 and into an amplifier burn-offload resistor 130.

Hot switch relay 125 may be any suitable solid-state high power switchcapable of switching a high power microwave energy signal. Hot switchrelay 125 receives the high power microwave signal from the signalgenerator 105 and amplifier 110, and passes the signal between theamplifier burn-off load resistor 130 or the circulator 135 withoutpowering down the signal generator 105 or amplifier 110. One suitabledevice is a JFW 50S-1552-N, which is a 150 watt 915 MHz dual polesingle-throw solid-state switch that can be powered by two DC supplylines and controlled with a single TTL signal line from a supervisorycontrol system or CPU 120. In use, the JFW 50S-1552-N allows the MRT 100to provide near instantaneous power (i.e. can provide nearly continuouspower with very rapid on/off capabilities) without creating amplifiertransients, by eliminating the need to power down the signal generator105 or amplifier 110.

At times, the MRT may provide two sources of electrical isolationbetween the microwave energy signal and the measurement devices. Forexample, the first source of electrical isolation may be provided by theelectrical isolation in the hot switch relay 125 between the output ofPosition A and the output of Position B. This electrical isolationprevents unacceptable amounts of energy from the high power microwaveenergy signal from being passed to the Position A output and to themeasurement system connected thereto. For example, at 915 MHz the JFW50S-1552-N switch (discussed above) provides about 45 dB of electricalisolation between outputs. The second source of electrical isolation isprovided by the transfer switch 140 and the electrical isolation betweenPort 4 and Port 2 of the transfer switch 140 discussed hereinbelow.

Continuous operation of the signal generator 105 and amplifier 110prevents the introduction of amplifier 110 transients into the microwaveenergy delivery system. To maintain continuous operation, the switchingtime between Positions A and B on the hot switch relay 125 should besufficiently fast to allow continuous operation of the signal generator105 and amplifier 110. For example, at 915 MHz the JFW 50S-1552-Nswitches between Position A and B in about 360 ns and between PositionsB and A in about 370 ns.

Amplifier burn-off load resistor 130 may be any suitable coaxialterminator capable of dissipating microwave energy while generating aminimal amount of VSWR, or reflective energy, over the bandwidth of thesignal generator 105. One such device is a 1433-3 50-ohm 250-wattcoaxial terminator sold by Aeroflex/Weinschel and intended for operationover the bandwidth of DC to 5 GHz. Over the entire bandwidth of the1433-3 the VSWR is less than 1.1.

Circulator 135 is a passive three port device that eliminates standingwaves between the hot switch relay 125 and the transfer switch 140.Circulator 135 passes signals received on Port A to Port B, signalsreceived on Port B to Port C and signals received on Port C to Port A.When hot switch relay 125 is in Position A, the microwave energy signalis passed from Port A of the circulator 135 to the transfer switch 140connected to Port B. Reflected energy from the transfer switch 140 orthe DUT 115, received on Port B, is passed to Port C and dissipatedthrough the reflected energy burn-off load resistor 142. Reflectedenergy burn-off load resistor 142 is similar in function to theamplifier burn-off load resistor 130 as discussed hereinabove.

Hot switch relay 125 and transfer switch 140, when switching fromPositions A to Positions B, appears as open circuits to the circulator135. During and after switching occurs, the circulator 135 clears thesystem of any residual power left in the system by directing theresidual power into the reflected energy burn-off load resistor 142.

In addition, when hot switch relay 125 switches from Position A toPosition B energy from dual directional coupler 145 and the DUT 115 isdirected through the transfer switch 140, to the circulator 135 and isdissipated by the reflected energy burn-off load resistor 142. With thehot switch relay 125 and the transfer switch 140 both in Position B theMRT 100 connects to the DUT 115 and performs active measurementsthereof. Interaction between the hot switch relay 125, the transferswitch 140 and active testing of the DUT 115 is further describedhereinbelow.

Transfer switch 140 provides sufficient electrical isolation between themeasurement system and the microwave energy delivery system. In PositionA, the high power microwave energy signal is received on Port 4, passedto Port 3 and to the directional coupler 145. The precision networkanalyzer 150, connected to Port 2 of the transfer switch 140, connectsthe transfer switch load resistor 155 on Port 1. In Position B, energyreceived on Port 4 is passed to Port 1 and dissipated by the transferswitch load resistor 155, and the precision network analyzer 150 on Port2 is connected to through Port 3 to the directional coupler 145 and theDUT 115. The transfer switch 140 maintains electrical isolation betweenPorts 4 and 2 (and electrical isolation between the high power microwaveenergy and the precision network analyzer 150) regardless of thetransfer switch 140 position.

In operation, microwave energy is switched to the amplifier burn-offload resistor 130 by the hot switch relay 125 before the transfer switch140 switches from Position A to Position B. As such, the transfer switch140 does not operate as a “hot switch” because it is not under a loadfrom the signal generator 105 or amplifier 110 when switching occurs.

One suitable device that may be used as a transfer switch 140 is aTNH1D31 coaxial transfer switch sold by Ducommun of Carson Calif. TheTNH1D31 displays less than 1.05 VSWR, better than 0.1 dB insertion lossand less than 80 dB electrical isolation for all states at 915 MHz. Thehot switch relay 125 switches out the high energy microwave energysignal before the transfer switch 140 transitions, therefore, transitiontimes for the transfer switch 140 are not critical. High-to-lowtransition times for the TNDH1D31 are about 75 ms and low-to-hightransitions times are about 25 ms.

Directional coupler 145 may be configured to operate like mostconventional directional couplers known in the available art. Asillustrated in FIG. 1, directional coupler 145 passes the high powermicrowave energy signal received on Port 1 to Port 2 with minimalinsertion loss. Energy reflected back from the DUT 115 and received onPort 2 of the directional coupler 145 is passed through the transferswitch 140 to Port B of the circulator 135. Energy received from thetransfer switch 140 on Port B of the circulator 135 is passed to Port Cof the circulator 135 and dissipated by the reflected energy burn-offload resistor 142.

Directional coupler 145 samples a small portion of each of the signalsreceived on Port 1 and Port 2 and passes a small portion of the signalsto Ports 3 and 4, respectively. The signals on Port 3 and 4 areproportional to the forward and reverse power, respectively. Themeasurement system measures the signal samples and provides themeasurements to the supervisory control system.

Directional coupler 145 samples a small portion of each of the signalsreceived on Port 1 and Port 2 and passes a small portion of the signalsto Ports 3 and 4, respectively. The signals on Port 3 and 4 areproportional to the forward and reverse power, respectively. Themeasurement system measures the signal samples and provides themeasurements to the CPU 120. The forward and reverse power measurementsfrom the directional coupler 145 are passively measured and the samplesmay be taken continuously or at a periodic sample frequency. Unlike thebroadband scattering parameter measurements, the directional coupler 145measurements are indirect measurements of the delivered energy. As such,the measurements from the directional coupler 145 are limited to thebandwidth of the microwave energy supplied to the ablation device 115from the signal generator 100 (i.e., feedback is fixed to the frequencyof the high power microwave energy signal). A single frequencymeasurements, or narrowband measurement, can be used to calibrateamplitude and phase at a single frequency. By calibrating and/orcompensating for the return loss to the antenna feedpoint and phase for‘open’ or ‘short’ we are able to obtain a characteristic representationof the antenna's behavior (i.e., a Smith Chart representation of theantenna behavior).

One suitable directional coupler 145 is a directional coupler sold byWerlatone of Brewster, N.Y. The directional coupler 145 may be a 40 dBdual directional coupler with 30 dB directivity and less than 0.1 dBinsertion loss from 800 MHz to 3 GHz.

DUT 115 includes a microwave ablation device that connects to Port 2 ofthe directional coupler 145 and may be any suitable microwave devicecapable of delivering microwave energy to tissue. DUT 115 may alsoinclude the tissue or surrounding medium in which the microwave ablationdevice is inserted or deployed.

Supervisory control system includes a central processor unit 120 (CPU)capable of executing instructions and/or performing algorithms,configured to receive one or more inputs and may be configured tocontrol one or more devices in the MRT 100. Inputs may include analoginputs, such as, for example, signals from the forward and reversecoupling ports, Port 3 and Port 4 of the directional coupler 145,respectively. Inputs may also include digital inputs, such as, forexample, communication with one or more devices (i.e., precision networkanalyzer 150).

CPU 120 may control one or more components of the MRT 100. The signalgenerator 105 may receive at least one of an enabled/disabled controlsignal from the CPU 120 and reference signal. Enable/disable controlsignal indicates that the MRT system is in a condition to receive amicrowave signal (i.e., the hot switch relay 125 and/or the transferswitch 140 are in a suitable position to receive a microwave signal).Reference signals may include the desired microwave frequency and a gainsetting. CPU 120 may also provide control signals to the precisionnetwork analyzer 150.

The functionality of the measurement system may be performed in the CPU120 and the precision network analyzer 150. As illustrated in FIG. 1,the CPU 120 receives the passive inputs of power measurements (i.e.,forward and reflected power signals from the directional coupler 145)and the precision network analyzer 150 performs active measurements ofthe DUT 115.

The measurement system may include other inputs, such as, for example,temperature sensors, cooling fluid temperature or flow sensors, movementsensors, power sensors, or electromagnetic field sensors. For example,an array of temperature sensors (not shown) configured to measure tissuetemperature surrounding the DUT may be connected to the CPU 120 or theprecision network analyzer 150. Tissue temperatures may be used togenerate an estimation of an ablation size or to generate an alarm orfault condition. Cooling fluid temperature or flow sensors may be usedto indicate proper operation of a cooled DUT 115.

In another embodiment, the CPU 120 or precision network analyzer 150 mayinclude all of the functionality of the supervisory control system,measurement system or any combination thereof. For example, in anotherembodiment of the present disclosure, as disclosed hereinbelow, theprecision network analyzer 150 may receive the passive inputs, performsthe active measurements and then report information to the supervisorysystem.

In yet another embodiment, the precision network analyzer 150 is part ofa modular system, such as, for example, a PXI system (PCI eXtensions forInstrumentation) fold by National Instrument of Austin, Tex. A PXIsystem (not shown) may include a chassis configured to house a pluralityof functional components that form the MRT 100 and connect over a CPIbackplane, across a PCI bridge or by any other suitable connection.

Precision network analyzer 150 of the measurement system may connect toPort 2 of the transfer switch 140. Precision network analyzer 150 may beany suitable network analyzer capable of performing scattering parametermeasurements of the DUT and/or determining loss information fortransmission system. Alternatively, precision network analyzer 150 maybe a computer or programmable controller containing a module, program orcard that performs the functions of the precision network analyzer 150.

In the embodiment in FIG. 1, precision network analyzer 150 is astand-alone device or member that is in operative communication withtransfer switch 140 and/or CPU 120. In another embodiment, thefunctionality of the precision network analyzer 150 may be an integralpart of the supervisory control system (i.e., a function of the CPU120).

Precision network analyzer 150 may function in a fashion similar to mostconventional network analyzers that are known in the available art. Thatis, precision network analyzer 150 may determine various properties thatare associated with the energy delivery system of the MRT 100, such as,for example, the transmission line, the DUT 115 or the mediumsurrounding the DUT 115 (i.e., tissue). More particularly, the precisionnetwork analyzer 150 determines at least one property or conditionsassociated with increases in reflected energy (i.e., properties that canbe correlated to reduction in energy transmission or decreases inoverall system efficiency, such as, a change in the characteristicimpedance (Zo) of at least a portion of the microwave energy deliverysystem). One suitable precision network analyzer 150 is a four portprecision network analyzer sold by Agilent of Santa Clara, Calif.

Precision network analyzer 150 may connect to the transfer switch 140through an attenuator 160 or other suitable protection device. Inanother embodiment attenuator 160 may scale the signal from the transferswitch 140 to one of a suitable power, current and voltage level.

Attenuator 160 may be a limiting device, such as, for example, afuse-type device that opens a circuit when a high power signal isdetected. Limiting device may appear transparent to the precisionnetwork analyzer 150 until the limiting device is hit with a high powersignal. One such device is a power limiter sold by Agilent of SantaClara, Calif., that provides a 10 MHz to 18 GHz broadband precisionnetwork analyzer input protection from excess power, DC transients andelectrostatic discharge. The attenuator 160 limits RF and microwavepower to 25 dBm and DC voltage to 30 volts at 25° C. at 16 volts at 85°C. with turn-on times of less than 100 picoseconds.

Limiting device may function as one of a fuse and a circuit-breaker typedevice. Fuse device may need to be removed and replaced after failurewhile a circuit-breaker type device may include a reset thatreinitializes the circuit breaker after a failure. Reset may be a manualreset or MRT 100 may include a reset circuit that is initiated and/orperformed by the supervisory control system or the like.

In an energy delivery mode, as illustrated in FIG. 1, the MRT 100 isconfigured to delivery energy to the DUT 115. The microwave energysignal from the signal generator 105 and amplifier 110 passed betweenthe hot switch relay 125 in Position A, the circulator 135, the transferswitch 140 in Position A, the directional coupler 145 and the DUT 115.The measurement system (i.e., the CPU 120) passively measures forwardand reflected energy at Port 3 and 4 of the dual directional coupler145. The precision network analyzer 150 is electrically isolated fromthe high energy microwave signal by the transfer switch 140.

In another embodiment of the present disclosure, electrical isolationbetween the ports of the transfer switch 140 allows a portion of thesignal at Ports 3 and 4 to pass to Ports 1 and 2 wherein the passedsignal is proportional to the high energy microwave signal from thesignal generator 105 and amplifier 110. The energy of the passed signalis either sufficiently attenuated by the transfer switch 140 to preventdamage the precision network analyzer 150 or the precision networkanalyzer 150 may be protected from excessive energy, (i.e., transientsand current or voltage spikes) by the attenuator 155, or alternatively,a limiter. The passed signal is shunted to a matched or a reference loadand dissipated, through the transfer switch load resistor 155 connectedto Port 1 and measured at Port 2 by the precision network analyzer 150.

Precision network analyzer 150 may be configured to passively measurethe forward and reflected voltages from the directional coupler 145 andthe energy waveform from transfer switch 140. Power parameters,including the magnitude and phase of the microwave signal, may beobtained or calculated from the measured signals, by conventionalalgorithms or any suitable method known in the available art. In oneembodiment, the forward and reflected measurements of power and phasecan be used to determine impedances and admittances at a given frequencyusing a Smith Chart.

In another embodiment, the impedance at the MRT 100 may be calculated asfollows: First, the forward and reflected voltages, Vfwd and Vref,respectively, are measured. Then, the voltage standing wave ratio (VSWR)may be calculated using the equation:

$V_{SWR} = \frac{V_{fwd} + V_{ref}}{V_{fwd} - V_{ref}}$

The magnitude of the load impedance (ZL) may be determined by firstcomputing the reflection coefficient, F, from VSWR using the followingequation:

${\Gamma } = \frac{V_{SWR} - 1}{V_{SWR} + 1}$

Then, based on intrinsic system impedance, the load impedance ZL is:

$Z_{L} = \frac{Z_{0}\left( {1 + \Gamma} \right)}{\left( {1 - \Gamma} \right)}$

Phase must be determined by the measured phase angle between the forwardand reflected signals.

Those skilled in the relative art can appreciate that the phase may bedetermined with calibrated or known reference phases (e.g., measurementswith a short or open at the antenna feedpoint) and with measured valuesof Vfwd and Vref. The magnitude and the phase of ZL can then becommunicated or relayed to the supervisory control system that may bedesigned to make adjustments to the MRT as discussed hereinbelow.

FIG. 2 displayed the MRT system state machine 200. The six states,defined as State S, State C and States 1-4, show the various states ofthe MRT 100 in FIG. 1 and are designated as 210-260, respectively. Theoperating states of the MRT 100 of FIG. 1 are determined by the positionof the two switches, the hot switch relay 125 and the transfer switch140, and the previous operating state of the MRT 100. In use, theoperation of the MRT 100 flows between the six states. Multiple statesend in the same switch orientation but are shown as different states toillustrate a unique control sequence. The utility of each state duringthe ablation cycle are described hereinbelow.

State S 210 is the Standby State 210 of the MRT. When power is removedboth switches 125, 140 default to this condition, therefore, thiscondition is also the failsafe position (i.e., the default conditionwhen power is removed or on power failure directs energy away from thepatient or medical personnel). As such, the system provides for safeoperation in the case of power failure, fault detection or when thesystem is not in use. A failsafe Standby State 210 also ensures that onstartup, transient power spikes or other potentially dangerous powersurges from the amplifier 110 are directed into the amp burn-off matchedload resistor 130 thereby protecting equipment downstream from the hotswitch relay 125.

State C 220 is the Calibration State 220 of the MRT. During theCalibration State 220 the hot switch relay 125 directs microwave powerfrom the signal generator 105 and amplifier 110 to the amp burn-off loadresistor 130 and the transfer switch 140 connects the precision networkanalyzer 150 to the DUT 115. One or more calibrations are performedduring this state. In one first calibration the precision networkanalyzer 150 may be calibrated to the DUT 115 reference plane, throughthe attenuator 160, transfer switch 140 and directional coupler 145, forbroadband scattering parameter measurements. A second calibration mayinvolve the measurement of line attenuation between the directionalcoupler 145 output ports and the DUT 115 reference plane. Determiningline attenuation may require a second calibration value that may beobtained by replacing the DUT with an ‘open’ or ‘short’ at the exactreference path length. Alternatively, a second calibration value may beobtained by operating the antenna in air and comparing this value with aknown value of the antenna operating in air. This attenuation value isused to calibrate power measurements at the directional coupler 145 topower delivered to the DUT 115. An initial broadband scatteringparameter measurement may be made during the Calibration State 220 tocapture the DUT 115 impedance within uncooked tissue.

State 1 130 begins post calibration or after State 4 260. During State 1130, the transfer switch 140 is activated which connects the DUT 115load to Port 2 of the circulator 140 and the precision network analyzer150 to the terminal switch load resistor 155. In State 1 230, the onlyhigh power signal present in the system is flowing between the signalgenerator 105, the amplifier 110, the hot switch relay 125 in Position Band the amplifier burn-off resistor 130. State 1 230 may include a delayto ensure that the transfer switch 140 has transitioned from Position Bto Position A. A fault condition in State 1 230 returns the system toState S 210, the Standby State 210.

State 2 240 begins after the transfer switch 140 has completed thetransfer switch's 140 switching cycle in State 1 230. A high controlsignal, delivered to the hot switch relay 125 from the CPU 120, directspower from the signal generator 105 and amplifier 110 through thecirculator 135, transfer switch 140, directional coupler 145 and intothe DUT 115. State 2 240 is the period during which an ablation isgenerated and generally represents the majority of system time. A faultcondition in State 2 240 returns the system to State S 210, the StandbyState 210.

State 3 250 ends a period of power delivery to the DUT 115 inpreparation for a precision network analyzer 150 scattering parametermeasurement. A low signal is presented to the hot switch relay 125directing power from the signal generator 105 and amplifier 110 into theamplifier burn-off load resistor 130. A period of clear line wait timeis added to the end of State 3 to allow the system to clear the circuitof high power signals. A fault condition in State 3 returns the systemto State S, the Standby State 210.

State 4 260 is initiated after the clear line wait time at the end ofState 3 250 expires. State 4 260 is initiated by activating the transferswitch 140. Activation of the transfer switch 140 restores the system tothe calibration configuration allowing the precision network analyzer150 to perform broadband scatter parameter measurement of the DUT 115.The only high power signals present in the system flow between thesignal generator 105, the amplifier 110, the hot switch relay 125 andthe amplifier burn-off load resistor 130. After the precision networkanalyzer 150 completes a measurement cycle the system leaves State 4260, re-enters State 1 230, and the MRT 100 repeats the cycle unless theablation cycle has ended or a fault occurs, in which case the systementers State S 210, the Standby State 210.

The MRT system state machine 200 essentially eliminates the risk of highpower signals from potentially damaging sensitive microwave equipment,such as, for example, the precision network analyzer 150. Additionalswitching and clear line delay times may be added into the system toensure this safety aspect of the system architecture.

FIG. 3 is a switch control state machine 300 for the microwave energydelivery, measurement and control system of the present disclosure. Withreference to FIG. 1, the position of the hot switch relay 125 isindicated in the upper timing diagram of FIG. 3 and the position of thetransfer switch 140 is indicated in the lower timing diagram. Ameasurement period 310 includes an energy delivery period 320, a clearline period 330, a first transfer transient period 340, a precisionnetwork analyzer sweep period 350 and a second transfer transient period360. The energy delivery period 320 is the period in which energy isdelivered to the DUT 115 and initializes the start of a new measurementperiod 310. The clear line period 330, which follows the energy deliveryperiod 320, provides a delay in which the standing waves and transientsin the system are allowed to dissipate through the circulator 135 andload 142 or the DUT 115. The first transfer transient period 340provides a delay to allow the transfer switch 140 to transition fromPosition A to Position B. The precision network analyzer sweep period350 provides time for the precision network analyzer 150 to performbroadband scattering parameter measurements. The second transfertransient period 360 provides a delay to allow the transfer switch 140to transition from Position B to Position A.

The time intervals of the timing diagrams in the switch control statemachine 300 of FIG. 3 are not necessarily to scale. For example, if thesystem is providing a continuous waveform, the energy delivery period320, or the “on-time” in which microwave energy is delivered to the DUT115, is a majority of the measurement period 310. The remaining portionof the measurement period 310, or “off-time”, is split between the clearline period 330, the first transfer transient period 340, the precisionnetwork analyzer sweep period 350 and second transfer transient periods360. The clear line period 330 and the first and second transfertransient periods 340, 360 may be fixed in duration and based on thespecific hardware used in the MRT system 100. The precision networkanalyzer sweep period 350 is based on one or more sampling parameters.Sampling parameters include the sweep bandwidth, the number of stepswithin the bandwidth, the number of samples taken at each step and thesampling rate.

The clear line period 330 must be sufficient in duration to allow alltransients in the system to dissipate after the hot switch relay 125switches from Position A to Position B. Transient, such as, for example,standing waves or reflective energy, may “bounce” between componentsbefore eventually being dissipated or shunted by the reflected energyburn-off load resistor 142, dissipated in the system 100, or expended bythe DUT 115. For example, the hot switch relay 125 may switch fromPosition A to Position B in as little as about 360 ns, thereby leavingenergy in the MRT 110 between the circulator 135 and the DUT 115. Theenergy may be sufficiently high to damage the precision network analyzer150 if energy is not dissipated.

After switching occurs energy remains in the system for an amount oftime. The amount of time is related to the cable length, or pathdistance, between the antenna and the hot switch relay 125. For atypical system using conventional cables having a transmission line witha dielectric value (∈) of about 2, the signal speed is about 1.5 ns/ftfor each direction. For example, a circuit and cable length of about 10feet between the DUT and the switch, a signal traveling away from thehot switch relay 125 would travel once cycle, or the 20 feet between thehot switch relay 125, the DUT 115 and back to the hot switch relay 125,in about 30 ns. Without dissipating the standing waves, the signal mayringing, or remain in the system, for as many as 5 cycles between thehot switch relay 125 and the DUT 115, or about 150 ns. Circulator maydissipate the standing waves to an acceptably low energy level in aslittle as one or two cycles between the DUT and the hot switch relay125. Transfer switch 140 remains in Position A until the energy hasdissipated to acceptably low energy levels.

In another embodiment of the present disclosure, the clear line period330 is variable and determined by measurements performed by theprecision network analyzer 150 or the CPU 120. For example, measurementsfrom the forward coupling port (Port 3) or the reverse coupling port(Port 4) of the directional coupler 145 may be used to determine ifenergy remains in the system. The hardware design, or at low microwaveenergy levels, the amount of transient energy remaining in the MRT 100after the hot switch relay 125 transitions from Position A to PositionB, may be minimal and may allow the clear line period 330 to be equalto, or about equal to, zero.

First transfer transient periods 340 provide a delay before initiatingthe precision network analysis sweep 350. The first transfer transientperiod 340 allows the transfer switch 140 to switch from Position A toPosition B before the precision network analyzer 150 begins thebroadband scattering parameter sweep.

Second transfer transient period 360 provides a delay before thesubsequent measurement period begins (i.e., the next energy deliveryperiod). The second transfer transient period 360 allows the transferswitch 140 to switch from Position B to Position A before the hot switchrelay 125 transitions from Position B to Position A and energy deliveryto the DUT 115 resumes.

During the precision network analyzer sweep 350, the precision networkanalyzer 150 determines broadband small-signal scattering parametermeasurements. The sweep algorithm, and the amount of time to perform thesweep algorithm, is determined by the specific control algorithmexecuted by the CPU 120. Unlike the passive forward and reflected powermeasurements, the measurements taken during the precision networkanalyzer sweep period 350 are active measurements wherein the precisionnetwork analyzer 150 drives the DUT 115 with a broadband signal andmeasures at least one parameter related to the signal (i.e., S11,reflection coefficient, reflection loss). The CPU 120 uses at least oneof an active measurement taken by the network analyzer 350 during thebroadband small signal scattering parameter measurements or a passivemeasurements from the directional coupler 145 in a feedback algorithmsto control further energy delivery and/or subsequent MRT 100 operation.

Energy delivery time, or “on-time”, as a percentage of the measurementperiod, may be adjusted. For example, the initial duration of the energydelivery may be based on historical information or based on at least oneparameter measured during the calibration or startup states, 220 210,discussed hereinabove. The “on-time” may be subsequently adjusted,either longer or shorter, in duration. Adjustments in the “on-time” maybe based on the measurements performed by one of the precision networkanalyzer 150 and the CPU 120, from historical information and/or patientdata. In one embodiment, the initial duration of an energy deliveryperiod 320 in the ablation procedure may be about 95% of the totalmeasurement period 310 with the remaining percentage, or “off-time”,reserved for measurement (“on-time” duty cycle approximately equal toabout 95%). As the ablation procedure progresses, the “on-time” dutycycle may be reduced to between 95% and 5% to reduce the risk ofproducing tissue char and to provide more frequent measurements. The“off-time” may also be used to perform additional procedures thatprovide beneficial therapeutic effects, such as, for example, tissuehydration, or for purposes of tissue relaxation.

In another embodiment of the present disclosure, as illustrated in FIG.4, the MRT 400 includes a signal generator 405, a microwave amplifier410, a directional coupler 445, a transfer switch 440, a terminator 455,an attenuator 460, a precision network analyzer 450 and a DUT 415. Inthe present embodiment, the precision network analyzer 450 performsactive and passive measurements of various system parameters of the MRT400.

MRT 400 includes a signal generator 405 and amplifier 410 to generateand supply a high energy microwave signal to the directional coupler445. In an energy delivery mode the directional coupler 445 passes thesignal to Port 2 of the transfer switch 440 and the transfer switch 440passes the signal to the DUT 415 through Port 3. In a measurement mode,the high energy microwave signal is passed to a terminator 455 connectedto Port 1 of the transfer switch 440. Precision network analyzer 450connects the first and second passive ports 451, 452 to the forward andreflected power ports, Ports 3 and 4, of the directional coupler 445,respectively. The active port 453 of the precision network analyzer 450connects to Port 4 of the transfer switch 440. Precision networkanalyzer 450 may connect to Port 4 of the transfer switch 440 through asuitable attenuator 455 as illustrated in FIG. 4 and discussedhereinabove.

In an energy delivery mode, the precision network analyzer 450 of theMRT 400 passively measures forward and reflected power of the highenergy microwave signal from the forward and reflected power ports,Ports 3 and 4, respectively, of the directional coupler 445.

In a measurement mode, the precision network analyzer 450 of the MRT 400actively performs broadband scattering parameter measurements byconnecting to the DUT 415 through Ports 3 and 4 of the transfer switch440. The precision network analyzer 450 drives the DUT 415 with a signalat a range of frequencies and measures at least one parameter related tothe DUT 415 at a plurality of frequencies.

Transfer switch 440 may be a single-pole, dual-throw coaxial switch thatprovides sufficient electrical isolation between Port 2 and Port 4 ofthe transfer switch 440 thereby preventing the high energy signal fromdamaging the precision network analyzer 450 in either the energydelivery mode, the measurement mode and while switching therebetween.Attenuator 460 provides sufficient signal attenuation to prevent thehigh energy signal from damaging the precision network analyzer 450.Alternatively, attenuator 460 may be a limiting-type device as discussedhereinabove.

In yet another embodiment of the present disclosure, as illustrated inFIG. 5, the MRT 500 includes a signal generator 505, an amplifier 510, aCPU 520, a hot switch relay 525, an amplifier burn-off load resistor530, a circulator 535, a transfer switch 540, an attenuator 560, aprecision network analyzer 550 and a tuner 565 positioned between thedual directional coupler 545 and the DUT 515. The tuner 565 may be atuning network or tuning circuit configured to match the impedance ofthe delivery system with the impendence of the DUT 515 or,alternatively, the tuner 565 is configured to match the impedance of theDUT 515 to the impedance of the delivery system. Tuner 565 may include avariable stub tuning network, a diode network or any other automatedtuning network or circuit capable of high power operation and having theability to match the DUT 565 impedance variations to the MRT 500 systemimpedance over the cooking cycle.

In calculating a tuner adjustment, the CPU 520 characterizes the tuner565 and removes the tuner 565 from the signal measured in the activemeasurement portion of the measuring cycle.

Tuner 565 may be incorporated into the DUT 515 wherein the CPU 520directs the tuner 565 to actively changes one or more properties of theantenna (not shown) in the DUT 515 such that the antenna impedanceappears to be about equal to a characteristic impedance, e.g. 50 Ohms.For example, the CPU 520 may instruct the tuner 565 to change theeffective antenna length or change one or more dielectric properties.

The CPU 520 may use feedback from the measurement system to optimizeenergy delivery to the DUT 515 during at least a portion of the ablationprocedure. Optimization may include: changing the frequency of thedelivered microwave energy to better match the impedance of the DUT 515,using the tuner 565 to adjust the output impedance of the MRT 500 tomatch the impendence of the DUT 515 or a combination thereof.

In one embodiment the supervisory control system uses a forward powermeasurement from directional coupler 545, a reverse power measurementfrom the directional coupler 545, or one or more broadband scatteringparameter measurements to optimize energy delivery.

FIG. 6 is a switch control state machine 600 for the microwave energydelivery, measurement and control system 500 illustrated in FIG. 5. Theposition of the hot switch relay 525 is indicated in the upper timingdiagram and the position of the transfer switch 540 is indicated in thelower timing diagram. A measurement period 610 includes an energydelivery period 620, a clear line period 630, a first transfer transientperiod 640, a measurement, CPU processing and tuner control period 650and a second transfer transient period 660. The clear line period 630 isafter the energy delivery period 620 and provides a delay in which thestanding waves and transients in the MRT 500 are allowed to dissipate.The first transfer transient period 640 provides a delay to allow thetransfer switch 540 to transition from Position A to Position B. Themeasurement, CPU processing and tuner control period 650 allows theprecision network to perform broadband scattering parametermeasurements, perform control algorithms in the CPU and to performadjustments to system tuning. The second transfer transient period 660provides a delay to allow the transfer switch 540 to transition fromPosition B to Position A.

The time intervals of the timing diagrams in the switch control statemachine 600 of FIG. 6 are not to scale. For example, the energy deliveryperiod 620, or “on-time” in which microwave energy is delivered to theDUT 515, is typically equal to a majority of the measurement period 610.The remaining portion of the measurement period, or “off-time”, is splitbetween the clear line period 630, the first transfer transient period640, the measurement, CPU processing and tuner control period 650 andsecond transfer transient periods 660. The clear line period 630 and thefirst and second transfer transient periods 640, 660, respectively, maybe fixed in duration and based on specific hardware in the system. Themeasurement, CPU processing and tuner control period 650 is based on thesampling parameter, processing time or tuner control time. Samplingparameters include the sweep bandwidth, the number of steps within thebandwidth, the number of samples taken at each step and the samplingrate. The CPU processing includes the execution of the tuner algorithmand the tuner control time includes a frequency adjustment, a tuneradjustment or any related system settling time.

The clear line period 630 must be sufficient in duration to allow alltransients in the system to dissipate after the hot switch relay 625switches from Position A to Position B. Transient, such as, for example,standing waves or reflective energy, may “bounce” between componentsbefore eventually being dissipated or shunted through the reflectedenergy burn-off load resistor 642, dissipated in the system, or expendedby the DUT 615. For example, the hot switch relay 625 may switch in fromPosition A to Position B in as little as about 360 ns, thereby leavingenergy in the circuit between the circulator 635 and the DUT 615. Theenergy present in the MRT 500 circuitry and the DUT 515 may besufficiently high to damage the precision network analyzer 550,therefore, the transfer switch 540 remains in Position A until theenergy has dissipated to acceptably low energy levels. As discussedhereinabove, the amount of time for the energy to dissipate is dependenton the circuit and cable length in which the standing waves must travel.In one embodiment (dielectric value, ∈, =2) the length of time is equalto:dissipation time=(2×distance*1.5 ns/ft)*safety factor;

wherein the distance equals the circuit length plus the cable length,safety factor equals 2 or 3 and the speed of 1.5 ns/ft is based uponapproximately ∈r=2 for typical transmission line cables

In another embodiment of the present disclosure, the clear line period630 is variable and determined by the precision network analyzer 550 orthe CPU 520 measurements. For example, measurements from the forwardcoupling port (Port 3) and the reverse coupling port (Port 4) of thedirectional coupler 545, may be used to determine if energy remains inthe system. The hardware design, or at low microwave energy levels theamount of transient energy remaining in the system after the hot switchrelay 625 transitions from Position A to Position B, may be minimal andmay allow the clear line period to be equal to, or about equal to, zero.

First transfer transient period 640 provides a delay before initiatingthe measurement, CPU processing and tuner control period 650. The firsttransfer transient period 640 allows the transfer switch 540 to switchfrom Position A to Position B before the precision network 550 beginsthe broadband scattering parameter sweep.

Second transfer transient period 360 provides a delay before thesubsequent measurement period begins (i.e., the next energy deliveryperiod). The second transfer transient period 660 allows the transferswitch 640 to switch from Position B to Position A before the hot switchrelay 525 transitions from Position B to Position A and energy deliveryto the DUT 515 resumes.

During the measurement, CPU processing and tuner control period, theprecision network analyzer 550 determines broadband small-signalscattering parameter measurements. The measurement algorithm isdetermined by the specific control algorithm used by the supervisorycontrol system and is similar to the precision network analyzer sweepalgorithm discussed hereinabove. The supervisory control system, or CPU520, the active measurements of the broadband small signal scatteringparameter measurements or the passive measurements from the directionalcoupler 545 in a tuning algorithm. The tuning algorithm checks for thepresence of a mismatch in impedance between the MRT 500, the DUT 515,and/or any combination thereof, and determines if an adjustment isnecessary to correct the impedance mismatch.

Energy delivery time, or “on-time”, as a percentage of the measurementperiod, may be adjusted. For example, the initial duration of the energydelivery may be based on historical information or based on at least oneparameter measured during the calibration or startup states, 220 210,discussed hereinabove. The “on-time” may be subsequently adjusted,either longer or shorter, in duration. Adjustments may be based on themeasurements performed by the precision network analyzer 550 and/or theCPU 520 or from historical information and/or patient data. In oneembodiment, the initial duration of an energy delivery period in theablation procedure may be about 95% of the total measurement period withthe remaining percentage, or “off-time”, reserved for measurement(“on-time” duty cycle approximately equal to about 95%). As the ablationprocedure progresses, the “on-time” duty cycle may be reduced to between95% and 5% to reduce the risk of producing tissue char and to providemore frequent measurements.

The “off-time” may also be used to perform additional procedures thatprovide beneficial therapeutic effects, such as, tissue hydration, orfor purposes of tissue relaxation. For example, tuning algorithm mayinitiate a re-hydration of tissue to reduce tissue impedance instead ofadjusting the frequency or re-tuning the MRT.

Another embodiment of the MRT is illustrated in FIG. 7 and is shown asMRT 700. MRT 700 includes a variable attenuator 770 that replaces thehot switch relay 125 in the MRT 100 in FIG. 1. In FIG. 7, the MRT 700includes a signal generator 705, a variable attenuator 770, an amplifier710, a CPU 720, a circulator 735, a load resistor 742, a transfer switch740, a transfer switch load resistor 755, an attenuator 760, a precisionnetwork analyzer 750, and a directional coupler 745 that connects to theDUT. The signal generator 705 that supplies a microwave frequency signalto the variable attenuator 770. Variable attenuator 770 includes avariable network or circuit that scales the signal from the signalgenerator 705 between 0% and 100% and provides the scaled signal to theamplifier 710. Amplifier 710 amplifies the signal by a fixed amount andprovides the signal to the circulator 735.

The MRT 100 in FIG. 1 controls the energy output (i.e., the power of themicrowave signal) by adjusting the output of the signal generator 105and/or the gain of the amplifier 110 (i.e., signal from the signalgenerator 105 amplified by the gain of the amplifier 710). In the MRT700 of FIG. 7, the energy output is controlled by one or more of thesignal generator 705, the variable attenuator 770 and the amplifier 710.The output energy of the MRT 700 in FIG. 7 is equal to the signalgenerator 705 output scaled by variable attenuator 770 attenuationpercentage and amplified by the gain of the amplifier 710.

With reference to the hot switch relay 125 in FIG. 1 and the variableattenuator 770 in FIG. 7, Position A of the hot switch relay 125 isequivalent to the variable attenuator 770 is Position A (i.e., a scalingfactor of 100%). In both FIGS. 1 and 7, Position A provides microwaveenergy to Port A of the circulator 135 and 735, respectively. Similarly,Position B of the hot switch relay 125 is equivalent to the variableattenuator 770 in Position B (i.e., a scaling factor of 0%). Position Bin both FIGS. 1 and 7, no microwave energy signal is provided to Port Aof the circulator 135 and 735, respectively.

The hot switch relay 125 in the MRT 100 of FIG. 1 includes a switch thatswitches between Position A and Position B and is capable of executingthe transition in a minimum amount of time to prevent transients orspikes in the waveform. The variable attenuator 770 in the MRT 700 ofFIG. 7 may includes an automated variable attenuator, such as, forexample, a rheostat-like circuit that does not switch but transitionsbetween Position A and Position B thereby generating fewer transientscompared to the switch in FIG. 1.

Attenuator activation time would be added to the dissipation timecalculation for safe switching and measurement.

In yet another embodiment of the present disclosure, the DUT includes aMRT calibration device configured to measure the length of thetransmission path from the antenna feedpoint to the directional couplerand each respective signal to the network analyzer. FIG. 8 is aschematic representation of an ablation device for use in calibrating amicrowave energy delivery, measurement and control system of the presentdisclosure.

As is known in the art, calibration of a microwave energy deliverysystem may be preformed by various calibration procedures. For example,one of a Short-Open-Load (SOL), a Short-Open-Load-Thru (SOLT), aShort-Short-Load-Thru (SSLT) and a Thru-Reflect-Line (TRL) calibrationtechnique may be used.

In one embodiment the system is calibrated with a Short-Open (SO)calibration technique. The SO calibration provides a determination ofthe relative performance of the DUT. The Short-Open calibrationtechnique is known in the art and is generally described hereinbelow.

The first step of the SO calibration is preformed by running themicrowave generator with a “short” at the output of the microwavegenerator (i.e., the coaxial cable connector). The second step of the SOcalibration is preformed by running the microwave generator with theoutput of the microwave generator “open”. The two steps of the SOcalibration, which is often referred to as “shifting a reference plane”allows the generator to analyze the system up to the output of thedirectional coupler. One shortcoming of performing this calibration byplacing the “open” and the “short” at the output of the generator isthat the calibration fails to account for any portion of thetransmission line beyond the microwave generator.

FIG. 8A illustrates the directional coupler 845 at the output portion ofa microwave generator 810 and a coaxial cable 820 that connects themicrowave generator 810 to an MRT calibration device 800 of the presentdisclosure. The MRT calibration device 800 includes a transmissionportion 830 and an antenna portion 840.

FIG. 8B illustrates the transition between the transmission portion 830and the antenna portion 840. Switching mechanism 850 is located adjacenton the proximal portion of the antenna under test 840 and on the distalportion of the transmission portion 830 of the MRT calibration device800. Switching mechanism 850 allows the system to perform an SOcalibration without replacing the DUT.

Switching mechanism 850 is further illustrated in FIG. 8C and includesan open circuit switch 850 a, a short circuit switch 850 b and a shortcircuit load 840 a.

The switching mechanism 850 in the MRT calibration device 800 allows thereference plane to be shifted to a point proximal the antenna therebyaccounting for a majority of the transmission path in the calibrationprocedure. An open circuit is first obtained by the open circuit control836 actuating the open circuit switch 850 a to an open position therebydisconnecting the inner conductor 832 and outer conductor 834 from theantenna under test 815.

A short circuit between the inner conductor 832 and the outer conductor834 through a short circuit load 840 a is obtained by the short-circuitcontrol transitioning the short circuit switch 850 b from Position A toPosition B. The short circuit load 840 a is a fixed load that replacesthe antenna under test 815. For example, in one embodiment the shortcircuit load 840 a is an antenna with a feedpoint equivalent to theantenna under test 815 thereby providing a known antenna response thatcan be used to calibrate the antenna under test 815.

With the short circuit switch 850 b in Position B the system yields aknown phase and amplitude of the reflected energy at the antenna feed.The antenna under test 840 is replaced with a short circuit load 840 bthat may include an equivalent path-length and/or an equivalent antenna.Energy provided to the short circuit load 840 a is reflected at theshort circuit load 840 a with a specific phase for the returned signal.

In test, the short circuit load 840 a returns energy at a first phaseand the open returns energy at a second phase. The short circuit load840 a places a voltage minimum at the short and full standing waves atevery λ/4 and 3λ/4 wavelengths on the transmission line proximal theshort circuit load 840 a. The open circuit 850 a places full standingwaves at the open and every λ/2 wavelengths on the transmission lineproximal the open circuit 850 a.

Using known open or short parameters and the present open and shortparameters the phase angle and returned power of the antenna may bedetermined. An active tuning circuit may use one or more of theseparameters to determine one or more system tuning parameters. Forexample, an active tuning circuit may be placed in the generator, thehandle of the microwave energy delivery device or any other suitablelocation. Active tuning circuit may determine a range of mismatch and/orprovide one or more calibration parameters to the system or may properlycalibrate to the antenna feedpoint.

For example, the antenna and/or the tissue may be behaving inductively(i.e., 50 Ω+20 Ωj wherein the positive 20 Ωj is inductive) orcapacitively (i.e., 50Ω−20 Ωj wherein the negative 20 Ωj is inductive).Calibrating to the antenna feedpoint the system can identify if theantenna and/or tissue is behaving inductively or capacitively. As such,the system can incorporate a matching network to offset the impedancemismatch.

In yet another embodiment of the present disclosure calibration isperformed by placing the antenna 940 of a microwave energy deliverydevice 915 in a calibration apparatus 900. Calibration apparatus 900includes a chamber 910 a configured to produce a known reflection andphase shift in an antenna 940 a when the antenna 940 a is placedadjacent the chamber 910 a. Calibration is performed by placing theantenna 940 a in a fixed position relative to the chamber 910 a anddriving the antenna 940 a with a predetermined signal. The microwavegenerator 905 a connects to microwave energy delivery device 920 a via acable 920 a and measures one or more parameters indicative of theperformance of the antenna 940 a and compares the measured parameterswith one or more predetermined parameters. The microwave generator 905 athen determines one or more calibration parameters or one or more tuningparameters for the antenna 940 a under test.

Chamber 910 a may be a cylindrical shaped chamber configured to receivethe antenna 940 a. Chamber 910 a may receive the distal end of themicrowave energy delivery device 915 a, including the antenna 940 a, asillustrated in FIG. 9A, or chamber 940 b may be configured to receivethe microwave energy delivery device 915 b, as illustrated in FIG. 9B. Apositioning mechanism or stop mechanism may provide consistent placementof the antenna in the chamber. Stopping mechanism may include a sensingmechanism to sense the placement in the chamber. Sensing mechanism mayprovide a signal to the system to indicate that the antenna is inposition. System, after receiving the signal from the sensing mechanism,may be configured to switch to a test mode in which the system drivesthe antenna with a predetermined microwave signal.

Calibration device 940 a may be configured as a stand-alone device asillustrated in FIG. 9A, configured to interface with the microwaveenergy delivery device (not shown), configured to interface with themicrowave generator, as illustrated in FIG. 9B or any combinationthereof. Calibration device 900 a may be a passive device that providesa load on the antenna 940 a wherein the antenna response 940 a to theload 900 a (the calibration device) is known to the microwave generator905 a.

With reference to FIGS. 9A-9B, calibration device 900 a, 900 b mayinclude a chamber 910 a, 910 b configured to receive at least a portionof the microwave energy delivery device 915 a, 915 b. Chamber 910 a, 910b may be configured to receive the antenna 940 a, 940 b or the antennaand a portion of the device transmission line 930 a, 930 b. Chamber 910a, 910 b is configured to position a microwave energy absorbing loadrelative to the antenna 940 a, 940 b.

In use, a clinician mates together the calibration device 900 a, 900 band the microwave energy delivery device 915 a, 915 b, respectively. Theantenna 940 a, 940 b of the microwave energy delivery device 915 a, 915b is positioned relative to calibration device 900 a, 900 b,respectively, and a calibration procedure is performed. The calibrationprocedure may be initiated manually, by the clinician, via a microwavegenerator input 906 a, 906 b or interface screen 907 a, 907 b or by aninput on the microwave energy delivery device (not shown).Alternatively, the calibration procedure may be automatically initiatedby the microwave generator 905 b. For example, placement of the antenna940 b relative to the load in the calibration device 900 b may trigger asensor 901 b or input to the microwave generator 905 b (not shown) and acalibration procedure may be automatically initiated.

In one embodiment, the calibration procedure includes the steps ofdriving the antenna with a microwave energy signal, measuring at leastone parameter related to the antenna and generating at least one antennacalibration parameter. The microwave energy signal may be apredetermined signal, a signal selected by the clinician or a signalselected for the specific antenna. The one or more parameters related tothe antenna may include one of forward power, reflected power, impedanceand temperature. The at least one antenna calibration parameter isrelated to the operation of the antenna, such as, for example, aparameter related to antenna tuning, a parameter related to theresonance of the antenna, a parameter related to antenna construction orany other suitable parameter related to microwave energy delivery.

Calibration device may be configured to interface with one of themicrowave energy delivery device or the microwave generator. Asillustrated in FIG. 9B, calibration device 900 b may connect to themicrowave generator 905 b via a cable 920 b. In another embodiment, thecalibration device 900 b may include a connector (not shown) thatinterfaces with the microwave energy delivery device 915 b when matedtogether. Connection between the calibration device 900 b and microwavegenerator 905 b or microwave energy delivery device 915 b may also beconfigured as a wireless connection. Connection may include one or moredigital or analog connections or may include a suitable communicationmeans, such as, for example, TCP/IP, OSI, FTP, UPnP, iSCSI, IEEE802.15.1 (Bluetooth) or Wireless USB. Calibration device 900 b mayprovide one or more parameters related to the calibration device 900 band/or the calibration procedure to one of the microwave energy deliverydevice 915 b and the microwave generator 905 b.

Calibration device 900 b may further include a positioner 902 b toposition the microwave energy delivery device 915 b in one or morepositions relative to the calibration device 900 b. As illustrated inFIG. 9B, positioner 902 b aligns with notch 916 b on the microwaveenergy delivery device 915 b such that the calibration device 900 b andmicrowave energy delivery device 915 b mate in position. Positioner 902b and notch 916 b are configured to position the antenna 940 b in adesirable position relative to chamber 910 b. Positioner may be anysuitable means of positioning the microwave energy delivery device 915 brelative to the calibration device 900 b such as, for example, a latch,a catch, a locking clam-shell, a clip, a locking or positioning pin, anunique shaped appendage and matching recessed portion configured toreceive the appendage and any other suitable positioning device.

Calibration device 900 b may further include a locking mechanism 903,904, 909 for locking the calibration device 900 b to the microwaveenergy delivery device 915 b. As illustrated in FIG. 9B, catches 904align with slots 909 when chamber 910 b is in a closed position. Slide903 actuates catches 904 within the slots thereby locking the chamber ina closed position. Any suitable locking mechanism may be used such as,for example, a clip, a latch, a pressed fit pin, a locking orself-closing hinge, a magnetic or electronic closure mechanism or anyother suitable locking mechanism. Slide 903 or other locking releasemechanism may be configured to be disabled when the antenna 940 b isactivated thereby preventing the calibration device 900 b from releasingthe microwave energy delivery device 915 b during calibration or energydelivery.

As various changes could be made in the above constructions withoutdeparting from the scope of the disclosure, it is intended that allmatter contained in the above description shall be interpreted asillustrative and not in a limiting sense. It will be seen that severalobjects of the disclosure are achieved and other advantageous resultsattained, as defined by the scope of the following claims.

What is claimed is:
 1. A microwave generator system comprising: amicrowave energy source configured to generate a microwave energysignal; a transmission line coupled to the microwave energy source andconfigured to carry the microwave energy signal from the microwaveenergy source to a microwave energy delivery device for treating tissue;a measurement system coupled to the transmission line; and a switchingnetwork coupled to the transmission line and configured to couple themicrowave energy delivery device to the microwave energy source for afirst period and to couple the microwave energy delivery device to themeasurement system for a second period after the first period.
 2. Thesystem of claim 1, further comprising an attenuator coupled between themeasurement system and the switching network, wherein the measurementsystem includes an active measurement system configured to measure aparameter of a microwave energy signal propagating through thetransmission line.
 3. The system of claim 2, wherein the activemeasurement system generates a broadband signal and measures broadbandscattering parameters during the second period, further comprising aprocessor configured to control the microwave energy source based on themeasured broadband scattering parameters during a period subsequent tothe second period.
 4. The system of claim 1, wherein the measurementsystem is a precision network analyzer.
 5. The system of claim 4,further comprising a dual directional coupler coupled to thetransmission line, wherein the measurement system includes a passivemeasurement system coupled to the dual directional coupler andconfigured to measure parameters of microwave energy signals propagatingthrough the transmission line.
 6. The system of claim 5, wherein thepassive measurement system measures forward and reflected voltages of amicrowave signal propagating through the transmission line, furthercomprising a processor configured to calculate a magnitude and a phaseof the microwave energy signal based on the measured forward andreflected voltages of the microwave energy signal.
 7. The system ofclaim 2, wherein the parameter of the microwave energy signalpropagating through the transmission line is selected from a groupconsisting of voltage, current, and impedance.
 8. The system of claim 1,wherein the switching network includes: a first switch configured toswitch a connection to the microwave energy source between the microwaveenergy delivery device and a first resistive load; and a second switchconfigured to switch a connection to the measurement system between afirst resistive load and the microwave energy delivery device.
 9. Thesystem of claim 8, further comprising a circulator coupled to thetransmission line between the microwave energy source and the switchingnetwork, and configured to transmit a microwave energy signal receivedfrom the microwave energy source to the switching network and totransmit a microwave energy signal received from the switching networkto a second resistive load coupled to the circulator.
 10. The system ofclaim 9, further comprising a third switch coupled to the transmissionline between the microwave energy source and the circulator, and coupledto a third resistive load, the third switch being configured to switch aconnection to an output of the microwave energy source between the thirdresistive load and the circulator.
 11. The system of claim 10, furthercomprising a variable attenuator coupled between the microwave energysource and the switching network.
 12. A method of controlling amicrowave generator, comprising: switching an output of the microwavegenerator to the output of a microwave energy source of the microwavegenerator; controlling the microwave energy source to generate amicrowave energy signal during a first period; switching the output ofthe microwave generator to an input of a measurement system of themicrowave generator; and controlling the measurement system to measure aparameter of a microwave energy signal during a second period after thefirst period.
 13. The method of claim 12, wherein the measurement systemis a precision network analyzer.
 14. The method of claim 12, wherein theparameter of the microwave energy signal is selected from a groupconsisting of voltage, current, and impedance.
 15. The method of claim12, further comprising: generating a broadband signal and measuringbroadband scattering parameters during the second period using an activemeasurement system of the measurement system; and controlling themicrowave energy source based on the measured broadband scatteringparameters during a period subsequent to the first period.
 16. Themethod of claim 15, further comprising calculating a magnitude and aphase of impedance based on the measured broadband scatteringparameters.
 17. The method of claim 12, further comprising: measuringforward and reflected voltages of a microwave energy signal propagatingthrough a transmission line using a passive measurement system of themeasurement system during the second period; and calculating a magnitudeand a phase of impedance based on the measured forward and reflectedvoltages of the microwave energy signal during the second period.